A multi-sensor research network for the advanced characterisation of solar radiation, aerosol properties and trace gases in the Alps

A research network for accurate and long-term solar and atmospheric observations was started in 2004 in the Aosta Valley, a pristine Italian mountain region in the north-western Alps (mean altitude >2000 m a.s.l.). The network, operated by the local Environmental Protection Agency, aims at the continuous monitoring of solar irradiance at the surface, and the detection of changes in atmospheric composition and climate in such a fragile mountainous area. Monitoring of solar radiation in the ultraviolet (UV) band is performed based on a designed ad-hoc network to take into account the different elevations and environmental conditions found in the investigated area. Indeed, three UV broadband instruments are deployed at Saint-Christophe (570 m s.l.m., semi-urban site), La Thuile (1640 m a.s.l, mountain station), and Plateau Rosa (3500 m a.s.l., glacier), thus allowing to assess the changes of solar irradiance with elevation owing to the coupled effect of lower absorption and scattering by the atmosphere, and higher surface albedo, at high altitudes. Spectral measurements of UV-visible irradiance in the range 290-500 nm are performed by a double-monochromator spectroradiometer (Bentham DTMc300F), located at the main facility, in Saint-Christophe (45.7422°N, 7.3570°E, WIGOS ID 0-380-5-1), and complying with the stringent S-2 specifications defined by the World Meteorological Organisation in the frame of the Global Atmosphere Watch Programme. The whole network is physically traceable to the SI units (Physikalisch-Technische Bundesanstalt scale) through the world reference solar spectroradiometer QASUME. All UV datasets have been recently re-evaluated and homogenised to account for the effect of temperature of the diffuser and for calibration problems and changes in the calibration scale (“level 2”). A pyranometer and a pyrgeometer are also installed at the same site to record the irradiance of the short-wave solar and long-wave terrestrial components of the electromagnetic spectrum. Besides, monitoring of trace gases concentrations and aerosol properties is an additional, fundamental component of the network. Columnar ozone, sulfur dioxide and nitrogen dioxide concentrations are retrieved by a MkIV Brewer spectrophotometer. Aerosol properties are thoroughly characterised both at the surface (mass concentration, size distribution in the range 180 nm – 18 µm, chemical speciation, absorption by black/brown carbon) and in the layers above (optical and microphysical columnar properties from a sun/sky POM-02 photometer, and vertical profiles from a Lufft CHM-15k automatic LiDAR-ceilometer). So conceived, the network allows to disentangle the effects of the different atmospheric components affecting the optical radiation reaching the surface. Recent efforts made to further improve the quality of the network data and to make them available to the scientific community will be discussed. Current investigations about the aerosol direct and indirect radiative effects and their impact on mountain climate will finally be highlighted, together with potential use of our multi-instrument dataset for validation purposes (e.g., satellite retrievals) over complex terrain and in presence of snow.